Synthetic-material molded bodies, which are simple to manufacture and of which the surface is protected from external influences, are required for a large number of applications. Film-insert molding technology has become established for the manufacture of such synthetic-material molded bodies in injection molding processes. It provides that the front surface of a molded body is initially preformed in two dimensions or three dimensions from a coated film and afterwards filled from behind or respectively back-injected with a synthetic-material melt.
Here, it is often desirable that the front side of the molded body is adequately protected from chemical and mechanical influences. This is often achieved in the prior art through a corresponding coating or lacquering of the surface. In order to avoid a wet coating of the finished parts, it is advantageous here that such a lacquering or coating is already applied to the film, which then runs through all further forming steps together with the film.
For such preformed films, there are two variants for further processing which place different requirements on the coating.
One variant is the so-called 3-D deformation. The film with the coating is strongly thermally deformed. It should be possible to deform the coating with the substrate strongly and into the smallest radii without cracks. It should be thermoplastic and, up to this moment, not yet cross-linked. After the deformation, the coating is finally hardened in a second technological step, through UV radiation and the cross-linking caused by it. In this manner, a front surface of a 3-D synthetic-material part of complex structure is provided, which is completely protected in all positions, even in strongly deformed positions. The film coatings which are suitable for this technology are referred to by the technical term “formable hardcoating”, namely a film coating which is initially adequately block resistant, but which can then be arbitrarily thermally deformed together with the substrate and, at the end, receives the properties of a protective layer through UV hardening. Synthetic-material films coated with it are therefore thermally deformable and receive a scratch-resistant and particularly solvent-resistant surface through the downstream treatment with UV radiation. The desired synthetic-material molded bodies are obtained after the back-injection of these film products with polymer melts (film insert molding).
Such a combination—block resistance and thermoplastic behaviour of the primary coating together with the large latent potential for UV cross-linking—is difficult to realise and, after the deformation, require a UV hardening step of the deformed film part, in which it must be taken into consideration that the uniform irradiation of the film surfaces is made more difficult by the three-dimensional shape. The process of 3-D deformation is therefore sophisticated but demanding. It requires specially designed, coated substrates and above all special technological equipment for the steps of deformation and UV hardening.
Alongside the synthetic-material parts and molded bodies molded in a demanding manner as described, there are a large number of synthetic-material molded bodies which are constructed in a less complicated manner and are accessible via so-called 2-D deformation.
Accordingly, for a plurality of applications, synthetic-material molded parts are required, which comprise a slightly curved front surface with somewhat curved edges. Here, it is particularly desirable that the surface of these parts is protected from mechanical and chemical influences via a film fitted with a protective layer. The manufacture of such parts is conventionally implemented in an injection molding machine in one step. Accordingly, a flat (2-D) cut-to-size piece of a coated film is placed into an injection molding mold and pressed with a synthetic-material melt at high temperature against a prefabricated, formed mold-wall under high pressure. A slight deformation of the film therefore occurs simultaneously in one step with the back injection. The synthetic-material part should then be ready. Further treatment steps, such as a hardening step of the coating of the film are not provided or desirable in this case. This means that the protective function of the coating on the film should already be fully built up before its use in such 2-D film-insert molding processes.
A high scratch resistance and a high solvent resistance can generally be achieved only with a high cross-linking density within the coating of the film. Such layers are generally hard, rigid, brittle and cannot be deformed together with the substrate without cracking, even at increased temperatures. A good protective effect of the lacquer coating and its formability are therefore opposites.
A coating agent which, as a coating for a film, ensures good scratch resistance and solvent resistance, but which is then adequately thermally deformable together with the film in a film-insert molding process, especially in an FIM-2-D process, is desirable but difficult to realise